In a wireless communications network, each device needs to transmit information by using a frequency resource. The frequency resource is also referred to as a spectrum. The spectrum may be categorized into a licensed spectrum and an unlicensed spectrum. The licensed spectrum is a frequency resource exclusive to some operators, and the unlicensed spectrum is a common frequency resource in the wireless communications network. A carrier on the licensed spectrum is referred to as a licensed carrier, and a carrier on the unlicensed spectrum is referred to as an unlicensed carrier. With development of communications technologies, an amount of information transmitted in the wireless communications network is increasingly growing. Preempting the unlicensed spectrum to transmit information may improve a data throughput in the wireless communications network, so as to better meet a user requirement.
In the prior art, to improve radio channel transmission reliability, as a main data retransmission means, hybrid automatic repeat request (HARQ) is widely applied. For example, after a first device sends data to a second device, the second device feeds back an acknowledgement/negative acknowledgement (ACK/NACK) message to the first device. The first device determines, according to information fed back by the second device, whether to transmit the data to the second device again. When required, the data is sent again. This process of data transmission is an HARQ process.
For example, in an licensed-assisted access using Long Term Evolution (LAA-LTE) system, when transmitting data by using the unlicensed carrier, a sending device cannot keep occupying a channel. After occupying the unlicensed carrier for a specific time, the sending device needs to preempt the channel again, and this causes interruption in HARQ retransmission. Therefore, a receive end can receive the data again only after the sending device preempts the channel again. How to achieve HARQ retransmission on an unlicensed carrier becomes an urgent problem to be resolved.